Speckle type noise is generally present in signals recorded when low-coherence interferometry is applied to characterize targets such as tissue samples that are surrounded by random media. The speckle noise is generally comprised of intensity contributions arising from multiple scattering loops which are collected by the optical system and which interfere during the temporal coherence interval. Because of the noise level, prior art types of low-coherence technique lack quantitative capabilities such as quantifying the optical contrast between the targeted region and the surroundings and are limited in use.
Optical noise present in low-coherence images is generally determined by the presence of multiple light scattering trajectories that have similar lengths as the ballistic component and that are collected by the measuring head. FIGS. 1A-1C show scattering paths having a total length such that the paths differ with less than the coherence length of illumination source will interfere and will generate the background component. FIG. 1A shows a Path A which refers to a single backscattered signal. FIG. 1B shows a Path which B refers to multiple forward scattering signals. FIG. 1C shows a path C which refers to multiple scattering signals. FIGS. 1A-1C show that due to the round-trip geometry of the paths, the actual penetration depths can be smaller than the actual depth of the target(see path C). Single backscattering contribution, paths of type A(FIG. 1A), and mostly forward scattering loops of type B(FIG. 1B) can have similar path lengths(within the coherence length of the source) and, therefore, contribute to the recorded signal. However, loops of type B(FIG. 1B) determine the beam spread and reduces the resolution.
The most difficult problem is to distinguish between paths of types A and B. The sizes of the scattering centers(scattering particles) in tissue are usually larger than the wavelengths. Accordingly, there is a strong forward scattering, which precludes the use of polarization-based methods to isolate these multiple scattering contributions.
Conventional approaches to reduce the optical noise in low-coherence techniques are to limit the measurements for targets at sufficiently small depths, to use low numerical aperture for the probe beam, to work at wavelengths such that the scattering is reduced, or decreases the coherence length. Besides reducing the number of multiple scattering events that are collected, the prior art approaches also affect the contrast, resolution, and penetration depth of a low coherence technique.
Based on a priori knowledge on the scattering, absorption, and structural characteristics, one can account for multiple scattering effects of path types B and C of FIG. 1. In applications where priori information such as particle size distribution, composition, and spatial location of scattering particles are known, scattering models can be used to derive the contribution of multiple scattering. The relative probability to generate paths of types A, B, and C from layers of thickness L.sub.c at the depth z(as shown in FIG. 1A), can be calculated if the optical characteristics such as cross sections, single scattering albedo and phase function, structural correlation's, layering, optical density of the surrounding medium are known. In FIGS. 1A-1C, source 40 can be an illumination light source, 10 is the air medium, 20 is the tissue being tested and 30 can be the subterranean target within the tissue, with L.sub.c is the coherence length of the light source, and Z is the depth within the tissue 20 to the target 30. The air-tissue interface shown in FIGS. 1A-1C, is only one example, interferometers can also be used in applications such as defect locations.
FIG. 2 illustrates how the multiple scattering contributions depend on the targeted depth z. The amount of multiple scattering contributions to the recorded signal depends not only on the depth value z but also on the coherence length L.sub.c and the optical characteristics of the medium between the interface and the targeted depth.
Referring to FIG. 2, probing the medium 20 at a higher depth actually enlarges the volume probed by OCT(optical coherence tomography). At higher depths, paths of types B and C become increasingly more probable adding their contribution to the background noise and decreasing both the axial and transversal resolution. Thus, the longer the depth the greater the noise. The complexity precludes a simple estimation of the multiple scattering background (noise level).
Various types of interferometers have been proposed over the years but fail to overcome all the problems described above. See for example U.S. Pat. No. 4,221,486 to Jaerisch et al.; U.S. Pat. No. 4,492,467 to Drain et al.; U.S. Pat. No. 5,469,259 to Golby et al.; U.S. Pat. No. 5,491,550 to Dabbs; U.S. Pat. No. 5,619,326 to Takamatsu et al.; U.S. Pat. No. 5,682,240 to Redlitz; U.S. Pat. No. 5,694,216 to Riza; U.S. Pat. No. 5,696,579 to Johnson; U.S. Pat. No. 5,716,324 to Toida; and U.S. Pat. No. 5,748,313 to Zorabedian.